On all userlists we have created in CB, the end user can search someone based on their country and state/province. The state/province field auto-populates based on what the user picks for country. When the user picks USA, then picks a state, the state variable is passed along. But any other country, and the state variable is not passed along.

Here is the resulting URL generated when Australia/Queensland is picked: /test-list?cb_country=AU
Here is the resulting URL generated when USA/Alaska is picked: /test-list?cb_country=USA&cb_state=AK

The HTML code generated for the state/province dropdown list for all countries looks valid. Here's Canada:
<select name="cb_state" id="cb_state" class="cbTooltip form-control" data-cbtooltip-tooltip="<p>Province/State of residence</p>" data-cbtooltip-title="Province/State:" data-hasqtip="3">
<option value="" id="cb_state__cbf0">No preference</option>
<option value="AB" id="cb_state__cbf1">Alberta</option>
<option value="BC" id="cb_state__cbf2">British Columbia</option>
ETC

and here's USA:
<select name="cb_state" id="cb_state" class="cbTooltip form-control" data-cbtooltip-tooltip="<p>Province/State of residence</p>" data-cbtooltip-title="Province/State:" data-hasqtip="7">
<option value="" id="cb_state__cbf0">No preference</option>
<option value="AK" id="cb_state__cbf1">Alaska</option>
<option value="AL" id="cb_state__cbf2">Alabama</option>
<option value="AR" id="cb_state__cbf3">Arkansas</option>

I've checked the cb_country and cb_state fields to make sure they are populating correctly. I also changed the order the countries appear in the dropdown to see if that had any effect, and it didn't. Any ideas?

The fields are being generated by queries.
SELECT * FROM #__usstates WHERE CO='[cb_country]' and ST != '--'; and SELECT * FROM #__country ORDER BY Ordering, Country; The state field updates on cb_country. The display setting in the userlist tab under CB Core Fields Ajax for both fields is Disabled.

Is there a standard PHP script that CB uses to process userlists? Or somewhere I can look on CB to see what PHP script userlists are processed through? Looking through our PHP scripts, I'm not seeing anything.

I'm not sure which sure mode I'm using. There's no way to search again or refine the results of the search on these lists. And the search is working for state when we chose USA as the country, just not any other country. I attached a screen shot of the simplified search I set up. I also don't see a "Display Empty Update On" setting in the CB Core Fields Ajax integration settings. I attached a screen shot of that page too. We're using CB 2.7.2.